As well known, a standard configuration for a gas turbine envisages a plurality of vanes solidly connected to a casing which surrounds a rotating shaft guided by blades mounted thereon. In particular, each vane comprises an airfoil which is connected to a vane platform, which is in turn retained into the external casing. As hot combustion gases pass through the casing to drive the rotating shaft, vanes experience high temperatures, and for such reason they need to be cooled. Typically, cooling configurations have a cooling medium entering the vane through the platform to the airfoil. In order to maximize the efficiency of the energy conversion process, the airfoil sections are relatively thin. In contrast, the platform sections to which they are attached are much thicker in order to provide suitable support for the airfoil.
FIG. 1 and FIG. 2 show a prior art design depicting a gas turbine vane in perspective and plan views respectively, the gas turbine vane being generally indicated with numeral reference 100 and comprising a vane airfoil 12, having a trailing edge portion 121, and a vane platform 200 including a hook portion 210. Furthermore, the vane platform 200 includes a wedge face pressure side 202 and a wedge face suction side 201 opposed thereto.
Making reference to FIG. 3, it is shown a perspective view of a portion of the gas turbine vane 10 of FIGS. 1 and 2 enclosed into the dashed box C. Not visible in the FIG. 3 is the wedge face suction side, opposed to the wedge face pressure side 202 of the vane platform 200 and the leading edge of the airfoil 12.
Making now reference to the following FIG. 4, in order to maintain proper cooling of the vane platform 200 a maximum surface is intended to be accessible for impingement cooling, especially for front stage vanes. The flow of the cooling medium is indicated with arrows A. Therefore vane hook portions 210 are shifted to extreme positions at upstream and downstream ends of the vane platform 200, thus forming a cavity, open towards the cooling air side. By positioning the downstream side hook portion 210 at the most downstream location, it almost lines up in radial direction with the trailing edge end 121 of the airfoil 12. As cooling is strictly required to ensure lifetime of the component, vane platform 200 is necessarily thick to allow proper internal cooling features. As a result, hook portion 210 close to airfoil trailing edge 121 results in a very stiff structure at the transition from airfoil trailing edge 121 to vane platform 200.
Such inflexible structure causes locally high stresses. Therefore, requiring a high amount of cooling air to maintain lifetime at reasonable levels having got a negative impact on the engine performance.
With reference to FIG. 5, it is shown a known solution to the aforementioned technical problem. In order to increase flexibility of vane platform 200, hook portion 210 is shifted inwards thus creating long overhangs 112. However, not all turbine configurations allow for such design, and, in any case, this solution causes a severe reduction of cooled area which may compromise lifetime for highly loaded parts.